Alzheimer's disease afflicts an estimated four million Americans and up to 600,000 cases of dementia are presented for diagnosis annually (Wall St. J., 1995). Patients with dementia must undergo a series of tests to eliminate other possible diseases to try to narrow the diagnosis to Alzheimer's. Such tests can take weeks and cost considerable sums of money. Although no effective treatment currently exists for most of these patients, quicker diagnosis of Alzheimer's will enable physicians and family members to properly care for patients.
The cause of Alzheimer's disease is not known. The disease is a genetically heterogeneous neurological disorder characterized by progressive dementia. Characteristic pathological features of Alzheimer's disease include the presence of senile plaques and neurofibrillary tangles (highly insoluble protein aggregates) throughout the brain, particularly in those regions involved with memory and cognition. The protein apolipoprotein E (APOE) has been found to be associated with the .beta. amyloid protein found in both the plaques and tangles. The gene for APOE is found on chromosome 19q13.2; it is polymorphic, consisting of three alleles, E2, E3, and E4. Between 40-50% of late onset Alzheimer's patients have at least one copy of the APOE4 allele, and APOE4 has been identified as a risk factor for Alzheimer's disease (Saunders et al., 1993; Strittmatter et al., 1993).
U.S. Pat. No. 5,434,050 relates to detection and quantitation of amyloid deposition in a patient, while U.S. Pat. No. 5,441,870 relates to detection and monitoring of a secreted amino-terminal fragment of .beta.-amyloid precursor protein, associated with Alzheimer's disease. PCT publication, WO 95/16791, the entire reference of which is incorporated by reference herein, relates to methods for the clinical determination of the risk of late-onset Alzheimer's disease or for the diagnosis or prognosis of Alzheimer's disease, the methods involving the determination of the number of copies of the APOE4 allele in a patient sample. WO 94/09155 relates to methods of diagnosing or prognosing Alzheimer's disease involving detecting the presence or absence of an apolipoprotein E type 4 isoform or DNA encoding APOE4 in a subject.
Apolipoprotein CI is a plasma protein of 6600 Da consisting of 57 amino acids (Shulman, et al., 1975). In the circulation, the Apo CI protein is associated with chylomicrons, as well as LDL and VLDL lipoproteins. The function of the protein is not understood, but it is known that it is produced primarily in the liver. Trace amounts are found in brain. Transgenic mice deficient in APOCI have an impaired receptor-mediated clearance of degraded lipoproteins. ApoCI has been reported to block ApoE-mediated uptake of .beta.-VLDL by hepatic receptors (Weisgraber, et al., 1990; Sehayek, 1991).
The gene for APOCI is located on chromosome 19 (19q12-19q13.2) within a 45 kb cluster of apolipoprotein genes. Each lipoprotein gene consists of 4 exons and three introns, suggesting a common ancestral gene (Lauer, et al., 1988). Upstream from the APOCI gene is that for APOE, and downstream are a pseudogene APOCI', and the APOCIV, and APOCII genes. The gene for APOCI is 3' to that for APOE, with 5500 bp separating the two genes.
The APOCI gene has three known polymorphisms: a BglI site at position 673 (Accession M20902 Genbank), a DraI site, and an HpaI site at position 175 (Frossard, et al., 1987; Frossard, et al., 1987; Nillesen, et al., 1990). The primary transcript starts at position 493.
It has been suggested that the E4 allele of the APOE gene is in linkage disequilibrium with the CI HpaI site of the APOCI gene(Chartier-Harlin, 1994). However, the relationship of APOCI as a clinical risk factor for Alzheimer's disease has not been investigated.
Many factors affect the association of a disease with a particular allele. For example, the level of linkage disequilibrium between a mutation and a disease is determined by the time passed since the mutation and the recombination frequency. Forces such as selection, mutation, and/or migration can change gene frequencies over time. Population stratification can also have major effects. In the search for the Huntington's gene, significant differences were found with associations of particular markers and several distinct populations. For example, families from the UK showed significant association of the disease with the marker D4S95, while Danish families did not (Andrew, et al., 1993). In a more recent study of gene mapping in which Caucasian and Japanese families with Werner's syndrome were analyzed, it was found that 5 markers on chromosome 8 had evidence of linkage disequilibrium in Caucasian families. The Japanese families had evidence of linkage disequilibrium to 4 additional markers (Goddard, et al., 1966). This indicates that there are probably several mutations in the two populations. Ethnic specific allele frequencies, multiple origins of a mutation, and sample size can all have profound effects on allelic association. Hence, observation that particular alleles share a linkage disequilibrium alone is insufficient and at times is an erroneous basis to assess risk or presence of disease.
Since the cause of Alzheimer's disease is not known, the disease and predisposition to it are difficult to diagnose. The identification of additional risk factors may assist in both diagnosis of Alzheimer's disease and in designing future therapy and/or preventive measures for Alzheimer's disease. Because of all of the above problems, known procedures are not completely satisfactory, and persons skilled in the art have searched for improvements.